1. Field of the Invention
The present invention relates to a fluorescence and phosphorescence detection device, a fluorescence and phosphorescence detection method, and a paper-sheet processing device equipped with a fluorescence and phosphorescence detection device capable of detecting fluorescence emission and phosphorescence emission that are excited on a paper sheet in order to recognize the paper sheet.
2. Description of the Related Art
Conventionally, a security mark having a certain optical property is used for authenticating a paper sheet. For example, such a security mark is prepared with a special material that does not emit light when irradiated with a visible light but emits light only when irradiated with an excitation light of a predetermined wavelength. By arranging such a security mark on a paper sheet, it is possible to authenticate the paper sheet based on an emission state of the security mark when the security mark is irradiated with the excitation light. Moreover, a plurality of such security marks having different emission properties can be provided on a single paper sheet. For example, one security mark may show fluorescence emission in which the security mark performs emission only while it is being irradiated with the excitation light, and another security mark may show phosphorescence emission. In the phosphorescence emission, the security mark continues emitting even after the irradiation of the excitation light thereon has been stopped, but the emission intensity gradually attenuates as time passes.
Japanese Patent No. JP4048121 discloses a method of detecting the phosphorescence emission from a security mark. This method involves to determine the presence/absence of the phosphorescence emission based on one emission intensity measured while the security mark is being irradiated with an excitation light and the other emission intensity measured after the irradiation of the excitation light is stopped. Japanese Patent No. JP5172066 discloses a method of signal processing that allows to determine the presence/absence of the phosphorescence emission with high precision. This method involves detection of the phosphorescence emission using previously prepared reference emission function corresponding to the phosphorescence emission, that is, normalizing the reference emission function and a measurement emission function obtained by measuring the actual phosphorescence emission, and comparing the normalized functions.
Even for the same fluorescence emission, depending on the composition of the ink that performs the emission, the wavelength band or the emission intensity of the excited fluorescence can be different. International Patent Publication No. WO2011/114455 discloses a method of detecting a plurality of emissions by using a single sensor having four photodetection elements. In this method, a visible light of a first wavelength band is detected in a first photodetection element among the four photodetection elements, and a visible light of a second wavelength band, which is different from the first wavelength band, is detected in a second photodetection element. Accordingly, two inks that excite different emissions can be distinguished based on the measurement results obtained for different wavelength bands in the first photodetection element and the second photodetection element. In this structure, the remaining third photodetection element and the fourth photodetection element detect a visible light of the entire visible wavelength band, including the first wavelength band and the second wavelength band. The measurement results obtained from the first photodetection element and the second photodetection element are corrected by using the measurement results obtained from the third photodetection element and the fourth photodetection element. This structure allows detection of each of the two inks with high precision.
However, in the above-mentioned conventional technology, it is difficult to measure with high precision both the fluorescence emission and the phosphorescence emission by one sensor. Specifically, even if the same excitation light is used, the emission quantities of the excited fluorescence emission differs from the excited phosphorescence emission greatly. Accordingly, if the conventional method that takes into account either the fluorescence emission or the phosphorescence emission is applied as it is, it is difficult to detect each emission with high precision. Therefore, there was a need for an inexpensive and small fluorescence and phosphorescence detection device capable of detecting the fluorescence emission and the phosphorescence emission with high precision.